Neutron generator tube having a sealed-in gas replenisher



3,344,299 NEUTRON GENERATOR TUBE HAVING A SEALED-IN GAS RELENIsHER Filed Jan. 6. 1964 Sept. 26, 1967 A J. E. BouNDEN 2 sneeis-Sneet 1 NNN.

wwf mM 3,344,299 NEUTRON GENERATOR TUBE HAVYING SEALEDl-IN GAS REPLENISHER Filed Jan. 6. 1964 Sepi- 25, 1957 J. E. BouwmanY l 2- Sheets-Sheet 2 United States Patent Oii ice 3,344,299 NEUTRON GENERATOR TUBE HAVING A SEALED-IN GAS REPLENISHER John Ellery Bounden, Baldock, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Jan. 6, 1964, Ser. No. 335,792 Claims priority, application Great Britain, Jan. 14, 1963, 1,561/ 63 2 Claims. (Cl. 313-61) This invention relates to apparatus for carrying out a nuclear reaction by bombarding a target material with high energy ions in an ion beam, the ions being produced in an ion source and accelerated across an acceleration gap which contains gas at substantially the same pressure as gas in the ion source.

Apparatus of the above type does not require a pumping means to keep the gas pressure low in the acceleration gap andas a-result the ion source and the acceleration gap lcan be contained within a common sealed envelope. This enables the apparatus to be easily portable and increases the safety of the apparatus since there is no point at which the envelope need be breached to operate the apparatus.

The apparatus has its prime use as a neutron generator. In this case the favoured nuclear reactions are the DD and the DT reactions and a gas replenisher is provided within the sealed envelope.

The attractions of neutron generators of the above kind as already proposed or constructed would be greater if their neutron outputs could be increased. Attempts to design high neutron output generators based on the known prior art have, however, met with difficulties. It has been found that the high neutron output could not be achieved by merely raising the power input to the ion source and improving the cooling facilities. A large proportion of the extra power consumed in the ion source -did not appear as a stronger ion beam. Furthermore the composition of the ion beam appeared to be adversely affected as shown by the fact that the neutron output itself was not raised by an amount corresponding to the increase in strength of the ion beam.

The above described difficulties can be Iunderstood as the effects of sputtering of metal due to the eect of the plasma. When a glass shield as known, spaced from a metal extractor electrode, is used at hi-gh powers the plasma diffuses through the aperture and attacks the material which is ostensibly shielded. Attempts to prevent the plasma diffusion by constricting the aperture produce the undesirable result that the extraction of ions from the plasma is reduced and the ionbeam does not therefore increase in power as desired. Sputtering is a most undesirable phenomenon in that` the sputtered deposited metal can lead to electrical breakdowns, and gaseous impurities in the metal can be released during the sputtering process itself.

It would be possible to improve the ion extraction from the plasma using a constricted aperture by applying a negative potential of about 2 kv. to the extractor electrode to repel the plasma `but this necessitates the use of an additional electrode inserted into the plasma to hold the plasma at the desired potential with respect to the extractor electrode. An additional electrode is a further complication in the construction of the apparatus and would normally require a further breach in the integrity of the sealed envelope. Furthermore with such a scheme the glass shield would have to be aligned with respect to the extractor electrode within closer tolerances than in this invention so as to prevent the ion beam impinging on the extractor electrode. The system would also be prone to electrical breakdown between the plasma and the 3,344,299 Patented Sept. 26, 1967 extractor electrode owing to the applied potential difference.

An object of the invention is to provide apparatus of the above type having a high rate of carrying out the nuclear reaction.

A further object of the invention is to provide improved apparatus of the above kind for carrying out the DD or DT reactions for the generation of neutrons.

Another object of the invention is the provision of a neutron generator of simple construction which is relatively free from sputtering troubles and yet which -does not necessitate the use of applied potentials to reduce sputtering, thereby having a low cost and high safety factor against gas leakage through the envelope.

The invention consists in apparatus comprising a sealed envelope, walls defining a zone within the envelope for the production of ions in a plasma, a target material within the sai-d envelope, an extractor electrode forming a partition between the said zone and the said tar-get material, the said extractor electrode having an aperture for passage of ions from the said zone to the said target material, a coating of vitreous material on the surface of the said extractor electrode facing the said zone, and connection means for applying an electrical field to accelerate ions in a direction from said extractor electrode to said target material to bombard the target material and produce a nuclear reaction.

The vitreous material is preferably a glass. This can be applied to the surface of the extractor electrode as a powder and subsequently fused.

In a preferred form of the invention the coating of vitreous material terminates a short distance from the envelope to leave `an uncoated area and an auxiliary shield of vitreous material is positioned between the extractor electrode and the said Zone for production of ions, to shield the said uncoated area from the plasma.

It is preferred to provide the extractor electrode with a probe portion having an aperture located within the plasma during operation of the apparatus.

An embodiment of the invention i-s illustrated in the accompanying `drawing in which p FIGURE l is a section of a neutron generator along A-A in FIGURE 2,

FIGURE 2 is a view of FIGURE 1 in the direction of the arrow Z, Y

FIGURE 3 is a section of another neutron generator having revised ion source arrangements the view being along B-B in FIGURE 4, and

FIGURE 4 is a view of FIGURE 3 in the direction of the arrow Y.

In FIGURE 1 a glass tube 1 is sealed to a metal disc 2 forming part of an extractor electrode assembly 3. Disc 2 contains a plurality of holes 4 which assist cleaning during assembly and gas return during generator operation. A hollow lmetal cone 5 has a base portion 6 of enlarged thickness and a tapering probe portion 7 having an aperture 8 and is brazed to disc 2. A groove 9 is cut in tapering portion 7 a short distance from the disc 2.

A thin coating of glass covers the surface 10 of tapering probe portion 7 between groove 9 and aperture 8 and does not extend into aperture 8. The inside of the aperture is bare metal to key the plasma thereto during operation of the generator. The coating is about 0.015 thick and is too thin to be shown on the drawing.

A glass shield 11 is sealed to disc 2.

Disc 2 is sealed to a glass tube 12 to which is sealed a coaxial metal tube 13. A target shield 14 is brazed to tube 13 by ange 15. A slot 16 is cut into shield 14 to allow visual inspection of the target, but this is not essential to the operation of the ygenerator and could be omitted.

A target 17 is brazed to a collar 18 which is mounted on a metal tube 19. Tube 19 is supported by an end collar 2t) mounted on a metal tube 21 which tube is sealed to a short glass tube 22 sealed to tube 13. A small clearance is maintained between target 17 and shield 14 to allow a small bias voltage to be applied between the two.

Tube 1 carries an arm 23 which is sealed to a metal tube 24. Two metal discs 25 and 26 are fixed in a metal sleeve 27. The discs each contain four small holes (not shown) which allow passage of gas but substantially prevent passage of ion plasma. The holes in one disc are kept out of alignment with the holes in the other disc. Items 24 to 28 form a plasma trap. A metal tube 28 is hel-d in sleeve 27 and is sealed to a glass arm 29 having a pinch end portion 30.

A gas dispenser having a cover 31 is supported on a stiff wire 32. Electrical leads 33 and 34 are connected to a hairpin heater concealed within cover 31. Cover 31 is filled with titanium deuteride powder and is closed by end caps 35 and 36 of porous nickel. When heated the powder gives off deuterium.

A second glass arm 37 carries items 24A to 28A forming a second plasma trap similar to the above mentioned plasma trap. A glass arm 38 terminates in a pinch 39 carrying a Pirani lgauge assembly 40.

A glass disc 41 is mounted on an arm. A coil 42 of copper tube is supported round tube 1 to apply RF. electrical field by inductive coupling to the interior of the tube 1. Alternatively two ring electrodes using capacitative coupling could be used.

A metal guard ring 43 shields from voltage stress the seal between tubes 12 and 13. A short arm 44 (FIG. 2) is sealed after evacuation of the generator.

To operate the tube the portion from disc 2 to collar 20 is immersed in oil and a tube for target coolant inserted into the space behind the target. The hairpin heater is actuated by current transmitted through wires 33 and 34 and deuterium is given off. The Pirani gauge 40 senses the pressure in the tube 1 and, by connections not shown, regulates the heating current so that the pressure is maintained in the range 10-15 microns.

A fan is switched on to blow cooling air over tube 1, and coolant liquid is circulated over the back of the target 17.

An R.F. oscillation of about 17 mc./s. (other frequencies are usable) is impressed on coil 42 to ionize the deuterium in the tube 1. This has the initial effe-ct of reducing gas pressure and time should be allowed to allow the pressure to rise by emission of more deuterium from the deuteride in dispenser tube 31.

A voltage of 110 kv. is then impressed across the ion acceleration gap formed by extractor electrode 3 and target shield 14, the shield 14 being negative with respect to electrode 3 and electrode 3 being at earth potential.

A voltage of 380 v. is applied between shield 14 and target 17, the target being positive with respect to the shield 14, in order to reduce emission of secondary electrons from the target. Emitted secondary electrons are stopped by glass disc 41.

Such a tube having a target of tritium absorbed in a layer of titanium deposited on a molybdenum target can produce continuously more than 109 neutrons each second for many hours without deterioration. A similar tube having a rare earth or yttrium deposited as absorbing layer on molybdenum produced 101 neutrons each second and its life was improved.

In FIGURES 3 and 4 the numerals have the same signiiication as they have in FIGURES 1 and 2. Groove 9 `of FIGURE 1 is now, however, dispensed with and so are the plasma traps 24-28 and 24A-28A.

For glass disc 41' of FIGURE 1 there is now substituted a molybdenum cup 41' fixed in metal end plate 42 to which plate arms 29 and 38 are sealed. Glass envelope 1 has an internal glass tube 43 integral therewith, the diameter of the tube being substantially the same as the diameter of cup 41'. Tube 43 prevents plasma reaching cup 41.

Cup 41' has a threaded ring 44 to which can be attached conduits for cooling fluid.

The revised arrangement of this generator result in improved stopping of secondary electrons.

I claim:

1. In a neutron generator tube comprising a sealed envelope divided by an apertured frustro-conical extraction electrode into a first zone adapted t0 4have an ioncontaining plasma produced therein and a second zone lcontaining a target adapted to be bombarded by ions extracted from said plasma through said aperture, said electrode projecting into said first zone, the provision on at least a central area `of the surface of said extractor electrode facing said first zone of a thin layer of vitreous material to shield said surface from the plasma, said coating extending substantially to the edge of said aperture but being excluded from the internal surface of said aperture and being sufiiciently thin to allow said internal surface to make contact with the plasma.

2. A neutron generator as claimed in claim 1 Wherein said layer is terminated to leave an uncoated annular area of the electrode surface facing said first zone between said layer and said envelope, and Icomprising an auxiliary shield of vitreous material having an aperture coaxial with the aperture of said extractor electrode but of larger diameter, said auxiliary shield being spaced from said electrode within said first zone to shield uncoated area from the plasma.

References Cited UNITED STATES PATENTS 2,831,134 4/1958 Reifenschweiler 313-63 2,998,523 8/1961 Muench et al 250-845 3,178,604 4/1965 Eklund 313-63 3,240,970 3/ 1966 Reifenschweiler 313-61 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. IN A NEUTRON GENERATOR TUBE COMPRISING A SEALED ENVELOPE DIVIDED BY AN APERTURED FRUSTRO-CONICAL EXTRACTION ELECTRODE INTO A FIRST ZONE ADAPTED TO HAVE AN IONCONTAINGING PLASMA PRODUCED THEREIN AND A SECOND ZONE CONTAINING A TARGET ADAPTED TO BE BOMBARDED BY IONS EXTRACTED FROM SAID PLASMA THROUGH SAID APERTURE, SAID ELECTRODE PROJECTING INTO SAID FRIST ZONE, THE PROVISION ON AT LEAST A CENTRAL AREA OF THE SURFACE OF SAID EXTRACTOR ELECTRODE FACING SAID FIRST ZONE OF A THIN LAYER OF VITREOUS MATERIAL TO SHIELD SAID SURFACE FROM THE PLASMA, SAID COATING EXTENDING SUBSTANTIALLY TO THE EDGE OF SAID APERTURE BUT BEING EXCLUDED FROM THE INTERNAL SURFACE OF SAID APERTURE AND BEING SUFFICIENTLY THIN TO ALLOW SAID INTERNAL SURFACE TO MAKE CONTACT WITH THE PLASMA. 